Replace spaces with underscores in module directory names

Rename module directories (module 6 → module_6, module 7 → module_7) to
fix Google Colab "Open in Colab" link compatibility. Update Colab badge
URL in Peng_Robinson_EOS.ipynb accordingly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

module_7/README.md

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+# Exporting notebooks to PDF
+
+The file `vdw_report.tex.j2` formats the LaTeX PDF output to use a specific filename. It makes additional changes to the format of the code blocks.
+
+To create the output, use the `nbeconvert` utility in `jupyter`:
+
+```
+% jupyter nbconvert van_der_waals_EOS.ipynb  --to pdf  --TemplateExporter.extra_template_paths=.  --template-file vdw_report.tex.j2 --no-prompt
+```
+

module_7/vdw_report.tex.j2

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+((*- extends 'latex/index.tex.j2' -*))
+
+% ---------- Remove code cell prompts ----------
+((* block input_prompt *))((* endblock input_prompt *))
+((* block output_prompt *))((* endblock output_prompt *))
+
+% ---------- Document metadata ----------
+((* block title *))
+\title{Van der Waals Equation of State}
+((* endblock title *))
+
+((* block author *))
+\author{Eric Furst}
+((* endblock author *))
+
+((* block date *))
+\date{\today}
+((* endblock date *))
+
+% ---------- Code font size ----------
+((* block packages *))
+((( super() )))
+\usepackage{fancyvrb}
+\fvset{fontsize=\footnotesize}
+((* endblock packages *))